The ability of resonant converters to achieve near lossless switching enables efficient high frequency operation. However, in applications with highly dynamic loads and/or isolation in the feedback path, implementation details practically limit the achievable frequency. For the case of isolated topologies, an optocoupler is used to provide isolation between the sensed output (voltage or current) of the secondary side and the controller on the primary. The controller of the resonant converter responds to the demands of the output to maintain regulation by adjusting the control variable of the primary side devices. Optocouplers are slow devices that limit the achievable bandwidth of the control loop. This contradicts one of the benefits of high frequency operation being the ability to increase loop bandwidth. Even in non-isolated applications, the feedback loop acts at the frequency of the primary side devices. With secondary side control, the loop operates on each half cycle of the switching period thereby effectively halving the response time. Further, certain resonant converters suffer from poor performance when subject to wide load variation. For example, the series resonant converter loses regulation at no-load under traditional variable frequency (VF) control. Secondary-side control can overcome this.
One secondary-side control technique is controlling the amount of resonant energy transmitted to the load. One conventional approach involves introducing a phase-shift to the gate signals of synchronous rectifiers in a center-tap secondary transformer system. However this approach allows reverse current to flow in the rectifiers, causing increased voltage ripple at the output. Another conventional approach involves using a full-bridge rectifier configuration for a single-winding secondary transformer. Two rectifiers are simple diodes, and the other two rectifiers are synchronous (controllable) rectifiers. However with this approach, at least one diode is always in the rectification path, limiting the achievable efficiency at full-load.
Another secondary-side control technique is exploiting the conduction difference of the MOSFET channel of the synchronous rectifier and intrinsic body diode. One conventional approach involves using a single modulation scheme, which has lower transient performance than a dual-edge modulation scheme. Another conventional approach involves using a dual-edge modulation scheme which results in the fastest achievable response. However, the control is course as there is only the option of one or two resistive drops or one or two diode drops for full-bridge and center-tap rectifiers, respectively.